Natural and Constructed Wetlands

A class of wastewater treatment technologies that mimic natural processes such as interaction of soil-micro-organisms with pollutants as well as interaction of plants and other life in natural settings with pollutants in wastewaters are called as Natural Treatment Systems (NTSs). Natural systems can broadly be classified into two classes: [I] intrinsic and [II] engineered systems (Chaturvedi and Asolekar, 2009). The intrinsic natural systems are typically the so-called natural water-ways and aquatic systems which can be further subdivided into two divisions, namely: self-supporting and stressed systems. A self-supporting system typically allows degradation of pollution without altering its own mechanisms and processes; for example, rivers and lakes polishing traces of biodegradable organic matter or treated sewage with the help of plants and microorganisms present in the system. It must be noted that the natural systems also concurrently process other biodegradable loads of pollution arriving to the system via other natural biogeochemical routes including the routine humification of natural organic matter.

The stressed natural systems, however, are usually characterized by their display of inability to cope with and degrade rather large amounts of pollution subjected to them (for example, rivers and lakes receiving large loads of sewage and wastewaters from urban or peri-urban communities). Natural wetlands may serve as buffer for storm water runoff. They may also act as a biofilter as well as trap sediment and remove pollutants including heavy metals from the contaminated surface runoff. Wetlands are considered delicate eco-systems because normally they are habitats for native and migratory wildlife.

It is well known that the engineered natural wastewater treatment systems including river banks, wet-zones and their modified versions such as constructed wetlands, waste stabilization ponds, sewage fed aquaculture ponds, duckweed ponds or algal bacterial systems are known to render quite effective environmental services by treating biodegradable carbonaceous matter and by separating suspended loads of particulates. However, it should be noted that, among these wastewater treatment systems not all are particularly effective in removing nitrogen and phosphorus. In spite of their limitations, Natural Treatment Systems (NTSs) have attracted attention of environmental engineers and scientists by the virtue their abilities of treating wastewater at phenomenally low Operation and Maintenance (O&M) costs. They have been favourably looked upon in the third world countries, especially because of their low power requirement.

In the light of shortage of water in several parts of India; communities are searching for the alternatives that are less power intensive and less expensive to provide some kind of primary and secondary treatment. The NTSs typically fill the gap in the sense that they need relatively low O&M costs and far low power to run them when compared with conventional primary, secondary treatment alternatives - especially such as activated sludge process, trickling filter or extended aeration system. Experiences of application of a variety of natural treatment systems in India have been highlighted by Arceivala and Asolekar (2006) in chapters 9 to 11, Chaturvedi and Asolekar (2009) and Starkl et al. (2010).

River bank filtration, aquifer recharges and soil aquifer treatment systems and constructed/Natural wetlands have particularly demonstrated more applicability in the context of developing economy such as India. The constructed wetlands can be effectively combined with advanced tertiary treatment alternatives and the resulting high quality treated effluent can be gainfully recycled into production and sanitation applications. The constructed wetlands are most prone to engineering adaptation and modular application and that is the reason why these natural treatment technologies have been chosen as the theme technology for in the present research project. Specifically, a pilot scale sewage treatment plant based on constructed wetlands will be constructed at IIT Bombay premises. The treated effluent generated from various modules of constructed wetlands can be subjected to the membrane processing units in the laboratory set-up and effective investigation of possibilities of combining constructed wetlands with advanced tertiary treatment unit can be undertaken - which is the main focus of the present research.